Chemical treatment of metal

ABSTRACT

A method for preparing metal surfaces for receipt of a coating such as a paint or adhesive (whereby increased coating adhesion and corrosion resistance is achieved) and the treated metal per se, are disclosed. The method comprises contacting the metal surface with various phosphinic and phosphonic acids.

United States Patent Vernon Paul Wystrach Wilton;

Francis Clyde Rauch, Stamford, both 01 Conn. 855,397

Sept. 4, 1969 Jan. 1 1, 1972 American Cyanamid Company Stamford, Conn.

Inventors Appl. No. Filed Patented Assignee CHEMICAL TREATMENT OF METAL[56] References Cited UNITED STATES PATENTS 2,230,371 2/1941 Keiser143/615 2,311,306 2/1943 148/615 3,224,908 12/1965 148/615 R 3,293,08812/1966 143/6. 15 R 3,468,725 9/1969 Uhlig 148/65 R FOREIGN PATENTS1,049,191 H1959 Germany 148/615 R Primary Examiner Ralph S. KendallAttorney- Frank M. Van Riet ABSTRACT: A method for preparing metalsurfaces for receipt of a coating such as a paint or adhesive (wherebyincreased coating adhesion and corrosion resistance is achieved) and thetreated metal per se, are disclosed. The method comprises contacting themetal surface with various phosphinic and phosphonic acids.

CHEMICALTREATMENT OF METAL BACKGROUND OF THE INVENTION The use ofvarious chemical materials in the treatment of metal surfaces to therebyrender them corrosion resistant is well known to those skilled in theart. For example, U.S. Pat. No. 1,798,2l8 describes a method wherebycertain molybdenum compounds are utilized whereas U.S. Pat. No.l,9ll,537, discloses the use of dicarboxylic and hydroxydicarboxylicacids for the same purpose. Phosphoric acid salts, (U.S. Pat. Nos.1,936,533; l,936,534; 2,952,699) phosphates, (U.S. Pat. Nos. 2,224,695;2,472,099; 2,769,737) and orthophosphoric acid-chlorinated hydrocarbonsolutions, (U.S. Pat. No. 2,789,070) have also been disclosed forsimilar purposes.

While these prior art techniques generally provide acceptable corrosionresistance, they usually fail in regard to the adhesion of surfacecoatings such as paints, varnishes, enamels, adhesives etc. thereto.Additionally, many of these antiquated systems are severely polluted bywater, i.e., when contacted with water they tend to peel, blister etc.

SUMMARY We have found that the adhesion of coatings to metals can bematerially increased or strengthened by first treating the metal with achemical material which is chemisorbed, i.e., chemically reacted with orabsorbed via strong bonds. In this manner, a foundation (integralchemical or chemically bound coating) is formed on the metal surface viareaction with the metal, which foundation is then more susceptible to anultimate or surface coating, such as a paint or adhesive, than materialsutilized in the past. Our method results in coatings which are moresecurely bonded or adhered to the foundation layer because the coatingis chemically bonded to the foundation and the foundation is chemicallybonded to the metal surface. That is to say, upon treating the metalaccording to our novel method, a reaction, as mentioned above, causes astrong bonding of the acid layer to the metal. There then remains free,for further reaction with a surface coating, a second reactive group inthe acid layer. This second reactive group then chemically combines withany surface coating applied thereto to produce a metal having a coatingtightly bonded thereto. Additionally, the corrosion resistance of themetal treated according to the present invention, with or without anextraneous coating on its surface, is at least as effective as knowncorrosion resistant systems.

DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS Our novelprocess comprises treating a metal surface, such as that of aluminum,steel, iron, copper, titanium etc. with a reactive phosphinic orphosphonic acid. The useful phosphinic acids have the formula and theuseful phosphonic acids have the formula /R2 HO OH tyl) phosphonic acid,bis(aminomethyl) phosphinic acid, bis(a-aminobenzyl phosphinic acid,(p-vinylphenyl) phosphonic acid (2 hydroxynaphthyl) phosphonic acid, bis(l-aminonaphthyl) phosphinic acid, (p-acryloylphenyl) phosphonic acid,(mercaptomethyl) phosphonic acid, bis( 2- mercaptoethyl) phosphonicacid, (diallyl) phosphinic acid, (2-butylene) phosphonic acid,(aminomethyl) phosphinic acid, (mercaptomethyl) phosphinic acid,(2-hydroxyethyl) phosphinic acid and the like. These specific compoundsare mentioned herein for purposes of illustration only and the list isnot meant to be inclusive of all possible useful compounds.

The phosphinic acids and phosphonic acids used herein are well known tothose skilled in the art as are methods for their production, such as,for example, those taught in U.S. Pat. Nos. 2,678,940; 2,717,906;2,874,] 84; 3,032,500 and 3,322,716, which patents are herebyincorportated herein by reference.

While not wishing to be bound by any particular theory, we believe thatthe =0 and OH groups of the phosphinic acids (or the OH groups of thephosphonic acids) react with the oxide layer formed on the surface ofthe metal to be treated. The existence of such oxide layers is wellrecognized by a cogent workers in the art. Such layers form uponexposure of the metal to the atmosphere. After the acid-oxide bonding iscomplete, the reactive groups of the acid foundation layer are then freeto react with the surface coating, i.e., paint, adhesive etc., therebychemically bonding the coating to the metal. The acid-oxide bonding ispseudochemical in nature and can be more accurately described as achemisorption of chelation of the acid by the metal.

The acid foundation layer may be applied to the metal surface, the metalfirst being thoroughly cleaned such as by degreasing withtrichlorethylene etc. or other common techniques, by immersing, dipping,painting, brushing, wiping, spraying etc. the metal article to betreated with solutions of one or more of said phosphinic or phosphonicacids, for a length of time such that the metal surface absorbs orreacts with a sufficient amount of acid. The metal is then merelyremoved from the solution and allowed to dry.

Additionally, the phosphinic or phosphonic acid can be applied to themetal by first incorporating the acid into the surface coating material,e.g., the paint or adhesive, and then applying the surface coating. Inthis manner, the acid can be added, for example, to the paint vehicle,and the paint then can be sprayed etc. onto the metal. Similarly, theacid can be added to one part of a two-part adhesive system and theadhesive can then be applied to the metal. In each instance, thereactive groups of the surface coating material will react with theappropriate groups of the acid while the acid itself reacts with theoxide on the metal surface, as more specifically described above.

The acid solution can comprise from about one part to about 50 parts ofthe acid per L000 parts of solvent, e.g., ethanol, methanol, water etc.The treatment is preferably conducted at room temperature althoughhigher or lower temperatures may be utilized, if desired. Completechemisorption of the acid onto the metal surface is generally achievedin from about 10 to about 20 minutes, the lower the acid concentration,the longer the reaction time necessary.

As mentioned above, the foundation layer of phosphinic or phosphonicacid affords a chemically available site whereby the reactive groups ofa paint layer may chemically react to thereby form a tightly adheringcoating layer. Examples of paints, adhesives or other coatings which maybe used include epoxy paints and adhesives, i.e., those containingchemically available CH--OH- groups, urethane paints and adhesives,i.e., those containing chemically available NCO groups, acrylic paintsand adhesives, i.e., those containing chemically available groups, vinylpaints and adhesives i.e., those containing dipped in a solution of theappropriate phosphinic or phosphonic acid for about l minutes.

After treatment, the aluminum electrode is placed in the cell and theappropriate electrical connections to the potenchemically available (,:HgrouPs and h like AS is tiostat made. The electrolyte is a citratebuffer of pH 4.6. The clear from the enuimmmnPfthe chemfcany avallablegroups initial potential of the aluminum electrode is then set at -l.0of the palms the free f volts, with respect to the reference electrode.The voltage the ayallable f of the Phosphlmc or Phosphomc scan is thenturned on. A sweep rate of 0.2 volts/minute in the F' layer prevlouslyappl'ed to the metal Surface- Thesefeac' positive direction is used andthe current developed is meagroups are represented RI and R2 fibove andm the sured on the recorder. The resulting polarization curve, voltcaseof epoxy and urethane adheswes would i age on the X-axis and the currenton the Y-axis may then by NH OH or SH groups, while in the case ofacrylic or analyzed to give the corrosion ram vinyl paints andadhesives, the group would be the unsaturated The method of analysis isthat used by Evans et 3|. L E]ec SUbStltUEnt, li'lClUdll'lg vinyl, etc.trochem. Soc.

TABLE I Corrosion Breakdown rate (mg./ potential Example Acid SolventConcentration dm day) (volts) Control.. No treatment 6.43 69 1Bis(hydroxymethyl) phosphinic acid Ethanol -.01% (10- M) 3.33 -.38 2a-Aminobenzyl) phosphonic acid.. er -.01% (10- M) .35 01 3(2-aminoethyl) phosphonic acid-.. d0. 01% (10* M) 24 +.16 4(l-aminoethyl) phosphonic ac1d d0 01% (10- M) .90 +.60 5 (Aminomethyl)phosphonic acid. .do -.0l% (10- M) 1.11 +.96 6 (l-aminobutyi) phosphonicacid ..d0 -.01% (10- M) .90

The paints can be applied in a condition such that the reaction concurswhile the paint vehicle evaporates or in a condition such that the paintmust more completely polymerize before it forms a useful coating. in thelatter case, if the functional group of the phosphinic or phosphonicacid is such that it initiates polymerization of the paint, the paintmay be applied in a prepolymer or semipolymer condition. An example ofsuch a treatment is illustrated by the use of an hydroxy groupcontaining phosphonic or phosphinic acid and an epoxy prepolymer. Inthis case, the hydroxy group both reacts with and cures (polymerizes)the epoxy prepolymer.

Additionally, we have found that our novel processing procedure can beutilized in conjunction with known corrosion resistance enhancingprocedures to obtain a pseudosynergistic effect. For example, we canimprove the corrosion resistance of metals treated according to ourinvention by first treating the metal with an inorganic chromate in amanner known in the art. Alternatively, the chromate (e.g. potassiumdichromate; chromic acid solution, etc.) may be incorporated into thephosphinic or phosphonic acid solution of our novel method beforetreating the metal according to our invention. in this manner thecorrosion resistance of the metal is increased without loss of theenhanced surface coating adherence mentioned above.

The pretreating technique and products of the instant invention findutility in military, industrial and consumer fields such as in thetreatment of aircraft and ship surfaces, cooling towers, heatexchangers, window screens, siding etc.

The following examples are set forth for purposes of illustration onlyand are not meant to be construed as limitations on the presentinvention except as set forth in the appended claims. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE I in determining the corrosion rate according to table I, amodified polarographic apparatus is used. The apparatus includes anappropriate experimental cell, a potentiostat and recorder. A threeelectrode system is used: (I) an aluminum electrode; (2) a calomelreference electrode and (3) a platinum counter electrode. The aluminumelectrode is 0.030 inch-diameter wire which is potted with acommercially available epoxy resin which in then sanded off so that theactual electrode area is the circular cross section ofthe wire. For thecontrol, the samples are merely untreated electrodes. For examples 1 6,the freshly polished aluminum electrode is EXAMPLE 7 Aluminum panels,3X5 inch, are degreased by dipping in benzene and further cleaned bydipping in hot l0 percent solution of a commercially available aluminumcleaner. The panels are then allowed to dry in air. One panel is thenimmersed in a solution of bis(hydroxymethyl) phosphinic acid in ethanol.After 15 minutes the panel is removed and allowed to dry.

The panel is then spray painted with a commercially available epoxypaint and allowed to dry and cure for 5 days at room temperature.

The painted panel is then subjected to a modified version of theCross-Hatch Tape Test" formulated by the National Coil CoatersAssociation. in the test, the painted surface is cut l0 times verticallyand I0 times horizontally with a razor blade, the scratch lines beingapproximately 5 mm. apart. Scotch cellophane tape No. 600 is appliedover the test area and rubbed with sufficient pressure to remove all airbubbles. The panel is allowed to set for 10 minutes and the tape is thenremoved sharply with a pull at right angles to the test surface. Avisual examination allows a reasonably accurate estimation of thepercent finish remaining on the panel in the test area.

The average results of tests conducted on panels treated according toexample 7, in addition to the results recorded utilizing different acidsaccording to the process of the present invention, are set forth intable II, below.

Following the procedure of example 1 except that (pacryloylphenyl)phosphonic acid is utilized in place of the acid used therein, a similarcorrosion resistant panel is produced.

EXAMPLE 13 Again following the procedure of example I a corrosion-resistant panel is produced utilizing (8-aminooctyl) phosphonic acid.

EXAMPLE l4 Utilizing the procedure of example 4 except thatbis(mercaptomethyl) phosphinic acid is employed, a corrosion resistantpanel is produced.

EXAMPLE 15 The procedure of example 7 is followed except thatbis(pvinylphenyl) phosphinic acid is utilized as the foundation layerand the surface coating is commercially available white acrylic paint. Apanel similar in surface coating retention to that of said example isobtained.

EXAMPLE 16 The procedure of example 7 is again followed except thatafter cleaning the surface of the metal panel with a degreasing agentand an alkali cleaning agent, 3 parts of bis(hydroxymethyl) phosphinicacid are added to 100 parts of the catalyst-curing agent package of acommercially available, two-package polyurethane adhesive composition.After blending the contents of the two packages together, the resultantmixture is applied to the clean aluminum panel and cured under therecommended conditions. The adhesive is bonded more tightly to the metalpanel than it is on a control specimen formed without the addedphosphinic acid.

EXAMPLE 17 The procedure of example 7 is again followed except that inplace of the epoxy paint used therein, a commercially available epoxyadhesive is used. The bonding of the adhesive to the metal is similar tothat of the paint of said example.

EXAMPLE 18 The procedure of example 1 is again followed except that(2-hydroxynaphthyl) phosphinic acid is used. Similar results areobserved.

EXAMPLE 19 The procedure of example I6 is followed except that (diallyl)phosphinic acid is utilized in place of the acid used therein and thesurface coating applied is in the form of an acrylic paint. The adhesionof the paint to the metal is superior to that of a test sample whereinno phosphinic acid is present.

EXAMPLE 20 A panel identical to that of example 14 is produced andcoated with a commercially available epoxy paint. The adhesion of thepaint to the metal panel is excellent.

EXAMPLE 21 The procedure of example 15 is again followed except that acommercially available vinyl paint is used in place of that paint ofsaid example and the metal used is steel. The adhesion of the paint tothe steel panel is superior to that of a panel coated without thefoundation layer of phosphinic acid.

EXAMPLE 22 The procedure of example I is again followed except that thealuminum is replaced by stainless steel. Similar results are recorded.

EXAMPLE 23 Following the procedure of example 5 except that the metalutilized is carbon steel, effective corrosion inhibition results.

EXAMPLE 24 The procedure of example 7 is again followed with replacementof the aluminum panels with similar sized sections of titanium sheet.The adherence of the epoxy paint to the phosphinic acid layer isexcellent.

EXAMPLE 25 The use of nickel sheet for aluminum of example l0 results in45 percent of the epoxy paint finish remaining after eight tests.

EXAMPLE 26 P O OH or a compound having the formula O\ /R2 H O OH whereinR 10R are, individually, aromatic (C -C or aliphatic (C -Cg) radicalsboth containing an NH OH or SH group and R is hydrogen or R,, andthereafter coating the resultant treated surface with a surface coatingcontaining groups chemically reactive with at least one of the groups ofsaid compound.

2. A method according to claim 1 wherein said compound is added as amixture with said surface coating reactive with said compound.

3. A method according to claim 1 wherein said compound isbis(hydroxymethyl) phosphinic acid.

4. A method according to claim 1 wherein said compound is(a-aminobenzyl) phosphonic acid.

5. A method according to claim 1 wherein said surface coating is apaint.

6. A method according to claim 2 wherein said surface coating is apaint.

7. A method according to claim 1 wherein said surface coating is anadhesive.

8. An article according to claim 1 wherein said metal is aluminum.

2. A method according to claim 1 wherein said compound is added as amixture with said surface coating reactive with said compound.
 3. Amethod according to claim 1 wherein said compound is bis(hydroxymethyl)phosphinic acid.
 4. A method according to claim 1 wherein said compoundis ( Alpha -aminobenzyl) phosphonic acid.
 5. A method according to claim1 wherein said surface coating is a paint.
 6. A method according toclaim 2 wherein said surface coating is a paint.
 7. A method accordingto claim 1 wherein said surface coating is an adhesive.
 8. An articleaccording to claim 1 wherein said metal is aluminum.